Process for making and using titanium oxide particles

ABSTRACT

Titanium oxide particles are prepared from the reaction of an aqueous basic solution with (1) an aqueous acidic titanium salt solution, or (2) an aqueous salt solution of a titanium salt and an aqueous salt solution of a dopant metal salt. In preferred embodiments, the particles may range in size from 0.1 to 10 microns (10-6 meters), and possess relatively high bulk density combined with high surface area.

This is a continuation-in-part of application Ser. No. 09/080,653 filedMay 18, 1998, U.S. Pat. No. 6,075,203, issued Jun. 13, 2000.

BACKGROUND OF THE INVENTION

The present invention relates to improved photovoltaic cells utilizingtitanium dioxide powders consisting of porous particles, ranging in sizefrom 0.1 to 10 microns (10⁻⁶ meter), which possess relatively high bulkdensity combined with high surface area.

Photovoltaic cells are devices which convert radiant photon energydirectly into electrical energy and are commonly used today in smallelectronic devices such as calculators and watches. These cells aremanufactured in a variety of configurations, but generally comprise alayered structure on a substrate. Conventionally, a transparentelectrically conductive material (known as an “electrode”) is firstdeposited on a substrate, onto which is deposited a semiconductormaterial, followed by one or more layers of semiconductor and/orinsulator material and/or conductive material. The last functional layeropposite the substrate must be a second electrode, i.e., transparentelectrically conductive material. In use, radiant photon energy shineson the surface of the photovoltaic cell causing electrons to movebetween the electrodes on the cell. The movement of electrons creates anelectrical potential difference and, therefore, the generation of anelectric current.

Titanium dioxide films are notable for their semiconductive propertiesand, as such, are useful as the semiconductive components ofphotovoltaic cells. However, conventional titanium dioxide has littleabsorbance of light in the visible region and often needs to be combinedwith or coated with a photosensitive material, such as a dye orchromophore, which absorbs light in the wavelengths emitted by the sun.

EP 407 182 discloses a multilayered photovoltaic cell in whichnanometer-sized titanium dioxide, prepared from colloidal solution, isutilized as a semiconductive layer. While some level of conversion isreported for the titanium dioxide alone, it is disclosed that niobiumdoped titanium dioxide gave superior results and it is suggested thatdye sensitized titanium dioxide and/or doped titanium dioxide is theoptimum choice. Given the recognized absorbance deficiencies of titaniumdioxide, much work has been devoted to the study of photovoltaic cellconfigurations and dyestuff additives to improve the absorbency oftitanium dioxide in the visible range. For example, WO 91/16719discloses doping the titanium dioxide with a divalent or trivalent metalto enhance the absorbency. U.S. Pat. No. 5,350,644 discloses a furthervariation on utilizing doped titanium dioxide wherein a multiplicity oflayers of titanium dioxide are formed on the substrate of thephotovoltaic cell with the requirement that a dopant be applied to theoutermost titanium dioxide layer and further that a photosensitizer beadditionally applied to the dopant-containing titanium dioxide layer.U.S. Pat. No. 5,441,827 describes photovoltaic cells using titaniumdioxide as the semiconductive material, wherein the colloidal particlesmaking up the titanium dioxide layer have a diameter which is smallerthan the diffusion length of the minority charge carriers. Such arestriction requires that the diameter be less than about 100 or 200nanometers (10⁻⁹ meter) and particles in the size range of 15 nanometerswere found to be the optimal size for photovoltaic application. See,e.g., Gratzel et al., “A low-cost, high efficiency solar cell based ondye-sensitized colloidal TiO₂ films”, NATURE, Volume 353, pages 737-738(Oct. 24, 1991).

The colloidally prepared titanium dioxide utilized in the art to datehas some well-recognized disadvantages due to the very small particlesize. For example, this nanoparticle titanium dioxide is very difficultto handle due to the low bulk density and tends to become easilyairborne requiring special safety precautions in handling. When storedas colloidal solution, the shelf life is short. This is due to theaggregation of TiO₂ particles which precipitates out of the solution.

This invention provides for an improved photovoltaic cell utilizing ahighly porous, high bulk density titanium dioxide in the size rangelarger than those disclosed in the art. The use of large, porous TiO₂particles allows for safe and easy handling of the particles and alsoretains the high photovoltaic efficiency of nanometer-sized TiO₂, acombination of the best of both materials.

Known solution based methods to produce titanium oxide powders tend tobe inefficient with respect to pounds of product per reactor volume,washing of the precipitated products to reduce anions to an acceptablelevel, and filtration time. The present invention improves upon all ofthese deficiencies of the prior art to provide processes with greatervolume efficiency to achieve more product per reactor volume. Thepresent invention also provides processes that require less water andless time to wash the products to acceptable ionic conductance levels.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of a typical dye-sensitized TiO₂photovoltaic cell in cross-section.

SUMMARY OF THE INVENTION

The present invention provides an improved photovoltaic cell, comprisinga bottom electrically conductive layer, at least one semiconductor layerand a top electrically conductive layer, wherein the at least onesemiconductor layer consists essentially of titanium dioxide particles,said particles having a size of 0.1 to 10 microns and being aggregatesof crystallites less than 100×10⁻⁹ meters in size.

The titanium dioxide utilized in this invention has an open porestructure wherein the average pore volume is at least 0.005 cc/g for any0.5×10⁻⁹ meter pore diameter size increment within a pore size range of2 to 10×10⁻⁹ meter.

DETAILED DESCRIPTION OF THE INVENTION

The photovoltaic cells which can benefit from the improvement of thisinvention include all known thin layer cells such as the Schottky diodetype, i.e., metal-semiconductor (MS) junction cells,metal-insulator-semiconductor (MIS) junction cells,semiconductor-insulator-semiconductor (SIS) junction cells, andheterojunction and homojunction cells. Processes for the manufacture ofthese cells are widely known and are described, for example, in U.S.Pat. No. 5,482,570, the disclosure of which is incorporated herein byreference.

The semiconductor layer in accordance with this invention can bedeposited by a conventional thick film screen printing technique. Forexample, thick film pastes are prepared by mixing the milled TiO₂ with aethyl cellulose/beta terpeniol polymer solution. A Hoover Muller is usedto compound the TiO₂/polymer into the viscous paste used for screenprinting. The substrates are patterned by using 360 mesh stainless steelscreens with bottom emulsions ranging from 5.8×10⁻⁶ meter to 3.8×10⁻⁵meter in thickness. All parts are printed using a lab scale handprinter. The prints are dried at 125° C. for 10 minutes then sintered at450° C. for 1 hr.

The titanium dioxide particles having the set of characteristicsrequired by this invention can be prepared in accordance with thefollowing procedure. Typically, the appropriate titanium dioxideparticles can be made by:

(1) preparing an acidic titanium salt solution, typically aqueous, of atleast one titanium salt of from about 10% to about 50% total titaniumsalt,

(2) precipitating the titanium dioxide particles by adding the acidictitanium salt solution to an aqueous basic solution containing at least20% base while maintaining the temperature at from about 20° C. to about95° C., after rapid initial heating of reaction and dilution, until a pHof from about 2 to about 4 is reached.

The selection of the base is important. Bases containing small inorganicions such as Li+, Na+, and K+ are not suitable since they are easilytrapped in the TiO₂ lattice and are difficult to remove by calcination.Bases containing organic cations such as NH4+ are preferred since theycan be easily washed out and can also be removed by calcination, even iftrapped.

To improve photovoltaic efficiency and allow for the use of thick filmtechniques in sample preparation, the agglomerate and particle size ofthe samples prepared by the above procedures is often reduced bymilling. The precipitated and dried oxide is first crushed to a −50 meshsize using a mortar and pestle. The oxide is calcined at 500° C. for 1hr and added to a #00 rubber lined ball mill ½ full of high density 10mm YTZ media. Isopropyl alcohol is used as the grinding solvent, this isadded to the mill containing the media and sample until the liquid levelcovers the media/sample in the mill. Typically 200 cc's is required formilling. The mill is than rolled at a speed of 180 rpm for a total of 18hours. The resulting slurry is dried on a hot plate over low heat for 4hours. Agglomerate size is reduced to a d50 of 0.5-0.7 μ after milling.

If it is desired to dope the titanium dioxide particles with some othermetal, one may do so by the following steps:

(1) preparing an aqueous salt solution of a titanium metal salt and anaqueous salt solution of a dopant metal salt,

(2) adding a major portion of the aqueous salt solution to an aqueousbasic solution containing at least 20% base and maintaining thetemperature at from about 50° C. to about 95° C. after rapid initialheating of reaction and dilution, to yield a first mixture,

(3) combining the remaining titanium metal salt solution with the dopantmetal salt solution to yield a second mixture, and

(4) completing the precipitation of the doped titanium dioxide particlesby adding the second mixture to the first mixture until a pH of fromabout 2 to about 4 is reached.

For each of the above processes, the solid particulate titanium dioxideis recovered by filtration, washed to a washwater ionic conductance ofabout 500 micromhos (microsiemens) or less and then dried and optionallycalcined.

The porous titanium dioxide particles prepared by the process of thisinvention can be used as an anode in dye-sensitized TiO₂ solar cell; aselectrochromic windows, or as an electrode in a lithium ion battery.

EXAMPLES

The basic configuration of a dye-sensitized TiO₂ photovoltaic cell isshown in FIG. 1, and comprises a sandwich-type structure 10 having afluorine-doped tin oxide glass (FTO) substrate 1, a TiO₂ layer coatedwith sensitizer dye 2; an electrolyte solution 3; and a platinum coatedindium-tin oxide glass (ITO) substrate 4. Typically, in constructingthis structure, TiO₂ solution (or paste) is either doctor-knifed orscreen printed onto a fluorine-doped tin oxide glass (1) to form athin-film of 5-10 microns thickness. This film is then sintered at ˜450°C. The sensitizing dye is deposited onto the TiO₂ particles by simplysoaking the film in an ethanol solution containing 3×10⁻⁴ Mcis-Di(thiocyanato)-N,N-bis(2,2′-bipyridyl-4,4′-dicarboxylicacid)-ruthenium (II)dihydrate dye. The dye-adsorbed TiO₂ film (2) isthen dried and sandwiched with a Pt-coated indium-tin oxide glass (4) toform the photovoltaic cell. The electrolyte solution (for example: 0.5 MLithium iodide and 0.04 M I2 in 4:1 ethylene carbonate/acetonitrile) isthen drawn into the cell. Sealing of the cell with adhesives completesthe fabrication process.

The ITO or FTO conducting glass substrates are cleaned before use with avapor degreaser containing chloroethene. The parts are held in thesolvent vapor for 20-24 hours. After removing from the degreaser, theparts are thermally cleaned by heating to 400° C. and holding for 1 hr.

Pt was deposited onto the indium-tin oxide glass by RF sputtering with aDenton Desk II sputtering unit.

The efficiency of the photovoltaic cell is determined by measuring theircurrent-voltage curves under the illumination of a 75 W xenon lamp. Theoverall efficiency of the photovoltaic cell, η_(global), is calculatedfrom the short-circuit photocurrent density, i_(ph), the open-circuitphotovoltage, V_(oc), the fill factor of the cell, ff, and the intensityof the incident light, I, using the following equation:

η_(global) =i _(ph) ·V _(oc) ·ff/I

Since the xenon lamp used is not a calibrated solar lamp, the measuredphotovoltaic efficiency does not correspond to the actual conversionefficiency under solar conditions. The measured values are usedrelatively for comparison purpose.

EXAMPLE 1

This Example substantially follows the procedure described by Gratzel etal., supra. Inside a glove box, 12.5 mL titanium isopropoxide was placedin a small dropping funnel. The funnel was capped and removed from theglove box and attached to a round bottom flask which contained a stirredsolution of 75 mL 0.1 M nitric acid in high purity water. The Timaterial was slowly dripped into the nitric acid solution under anitrogen blanket and with vigorous stirring. When everything was added,the solution was heated and stirred at 80° C. for >8 hours to give aclear solution. The solution was then filtered to remove dust and anyparticulates.

The solution volume was adjusted to 70 mL with water and then autoclavedwithout shaking at 235° C. for 12 hours. The milky white slurry wasthoroughly mixed by ultrasonication and then rotovapped to a finalvolume of 35 mL at room temperature. This white slurry was used forscreen printing of TiO₂ films after the addition of 1 gpolyethyleneglycol (mol. wt. 14,000) as a viscosity enhancer/binderprior to the film deposition.

Deposited films on FTO were sintered at 450° C. for 1 hour in flowingair prior to dye impregnation. Photovoltaic cell was constructed withthese TiO₂ using procedures described before. The efficiency wasmeasured to be 2.2% at a light intensity of 25 mW/cm² from a xenon lamp.(E88785-101). This cell is used as reference.

EXAMPLE 2

In this example, relativity large particle size pigmentary TiO₂ preparedusing a conventional chlorine oxidation process were used. In thechlorine oxidation process, purified TiCl₄ is burned in O₂ along with anadditional gas such as CO which is used to increase and maintain thetemperature of the reactants in the preliminary stage. Nucleation in thereactor is necessary to promote the formation of pigmentary particles.This is achieved by introducing a small amount of water vapor to theoxygen stream or by the combustion of hydrocarbons. Additionally, smallamounts of SiCl₄ and PCl₃ are added to the gas stream to suppressformation of the rutile phase. Because the growth rate is very rapid,the gases carrying the pigment are rapidly cooled to minimize furthergrowth. Separation of the pigment then follows with the chlorine beingcompressed and condensed to a liquid for further use. The pigment isthen washed to a neutral pH while heating with steam to remove anyadsorbed chlorine.

The final step in the process is to reduce particle and agglomerate sizeby media milling the washed pigment. The final TiO₂ has an average (d50)particle size of 410 nm in anatase form.

A photovoltaic cell was constructed with these TiO₂ using proceduresdescribed above. The efficiency was measured to be 0.04% at a lightintensity of 25 mW/cm² from a xenon lamp. This example demonstrates thatconventional large TiO₂ particles (that is, non-porous) are noteffective for photovoltaic application.

EXAMPLE 3

a. Preparation of “cut” TiCl₄ Solution

As a first step it was necessary to make an aqueous titaniumtetrachloride solution by carefully reacting pure TiCl₄ with deionized(DI) water. A significant number of precautions must be taken so as toaccomplish this reaction safely, due to the hazardous nature of handlingTiCl₄ and the strong exothermic reaction that occurs when it comes incontact water. Diluting TiCl₄ with water results in the formation oftitanium oxychloride, but for the purposes of these discussions theaqueous TiCl₄ solution will be referred to as “cut” TiCl₄.

The dilution of TiCl₄ with DI water was carried out in a 4-neck 3 literround bottom glass flask. A greased glass rod with a Teflon® coatedpaddle was inserted into the middle neck and attached to a lab motor. Agreased condenser was inserted into a side neck and fitted with a rubberseptum at the top through which a syringe was inserted to provide anitrogen gas purge during the reaction so as to keep air moisture in theglass flask to a minimum. A Teflon® taped dropping funnel, with a sidearm, was inserted into a third neck, which would later be charged withTiCl₄ solution and used to feed that solution into the DI watercontained in the glass flask. A greased thermometer was inserted intothe small side neck of the flask so that the temperature during thereaction could be monitored.

The glass flask was secured by clamps to a pole and lowered abouthalfway into a plastic bucket containing ice and water. 1000 grams ofchilled DI water was charged to the glass flask. Agitation of the waterand a nitrogen gas purge were both started. The one liter glass bottlecontaining 99.9% TiCl₄ solution, obtained from Aldrich Chemical CompanyInc. (cat. #20,856-6), had been chilled on ice for about 30 minutesbefore opening so as to minimize the fuming. When the chilled bottle wasopened, the TiCl₄ solution was poured into the dropping funnel with theaid of a glass funnel. Approximately 450 grams of TiCl₄ solution wasadded and the dropping funnel was then loosely capped by inserting arubber septum. The septum was not used to seal the funnel opening butonly close it up and allow the incoming nitrogen gas purge a place toescape. The dropping funnel was then opened and the TiCl₄ solution wasslowly added to the chilled water in the glass flask while maintainingagitation. The rate of addition was varied so as to maintain a liquidtemperature in the flask between 10 and 40° C. As expected, strongfuming in the flask from the TiCl₄ and water reaction resulted in athick cloud of vapor, however, this fuming was contained mainly in theflask as the chilled water condenser functioned properly and only atrace amount of vapor was venting from the loosely fitted rubber septumat the top of the dropping funnel.

Twice during this reaction, additional TiCl₄ solution was charged to thedropping funnel so that a total of 1107 grams was added. After the TiCl₄addition was complete, 108 grams of DI water was used to rinse down theside walls of the glass reaction vessel. The total amount of DI wateradded was 1108 grams. The clear yellow solution of “cut” TiCl₄ had anobserved weight of 2171 grams (theoretical weight=2215 grams). Thedifference of 44 grams is attributed mainly to an HCl:water vapor lossduring the course of the reaction. The theoretical concentration ofTiCl₄ in the “cut” TiCl₄ solution, assuming no mass loss, is 50%. Theactual concentration of the solution was checked by oven drying and thenashing a small sample of the solution in a vented furnace at 600° C. for15 minutes. The observed concentration of TiCl₄ was 47.5% based on molesof TiO₂ obtained from the ashing experiment.

b. Porous TiO₂ Particles from the NaOH Route

Seventy-five grams of 50% aqueous NaOH was charged to a 400 ml beakerand stirred with a Teflon® coated paddle that was attached to a labmotor. A dropping funnel was charged with 105.8 grams of a “cut” TiCl₄solution made in accordance with the above procedure and having anominal TiCl₄ concentration of 40%. The “cut” TiCl₄ solution was slowlyadded to the aqueous NaOH solution taking approximately 20 minutes tocomplete the addition. The final pH of the slurry was 3 and the maximumtemperature observed during the reaction was between 70 and 90° C. Theslurry was transferred to a Buchner funnel (11 cm in diameter) withfilter paper and washed with approximately 3 liters of DI water down toan ionic conductance of 20 micromhos. The total wash time was 5 hours.

The bulk density as measured from mercury intrusion data was 0.40 g/ccand the B.E.T. surface area was 404 m²/g. Using nitrogen intrusion data,the calculated BJH cumulative desorption surface area of pores between1.7×10⁻⁹ meters (17 angstroms) and 300×10⁻⁹ meters(3,000 angstroms) was519 m²/g and the BJH cumulative desorption pore volume of pores betweenthat same range was 0.69 cc/g. The crystalline powder phase wasidentified as 100% anatase having a crystallite size of 4.3'10⁻⁹ metersbased on X-ray line broadening of the (101) peak. The particle sizedistribution parameters, d₁₆, d₅₀, and d₈₄ were 1.0, 3.0, and 12.5microns respectively.

c. Porous TiO₂ Particles from the NH₄OH Route

A 400 ml beaker was charged with 142 grams of NH₄ 0H (28-30%), which waschilled using an ice bath, and stirred using a Teflon® coated paddlestir attached to a stirring motor. A peristaltic pump was used todelivery 195 grams of TiCl₄:DI water (40/60 wt.%) at a rate of 15cc's/min to the rapidly stirring NH₄OH.

The end pH of the precipitation was 3 and the maximum temperatureobserved during the reaction was between 60-65° C.

The viscous slurry was filtered using a glass fritted funnel and washedwith 10 liters of DI water to a ionic conductance of 7 micromhos. Thewashed and filtered oxide was dried at 125° C.

The bulk density as measured from mercury intrusion data was 0.64 g/ccand intrusion volume was 1.03 g/cc. The B.E.T. surface area was 394 m²/gand the crystalline powder phase was identified as 100% anatase.

d. Milling of the Porous TiO₂ Particles

The precipitated and dried oxide from Example 3b or 3c is first crushedto a −50 mesh size using a mortar and pestle. The oxide is calcined at500° C. for 1 hr and added to a #00 rubber lined ball mill ½ full ofhigh density 10 mm YTZ media. Isopropyl alcohol is used as the grindingsolvent, this is added to the mill containing the media and sample untilthe liquid level covers the media/sample in the mill, typically 200 cc.The mill is than rolled at a speed of 180 rpm for a total of 18 hours.The resulting slurry is dried on a hot plate over low heat for 4 hours.Agglomerate size is reduced to a d50 of 0.5-0.7μ after milling.

EXAMPLE 4

The TiO₂ used in this cell was made as per Example 3c and milledaccording to 3d. Standard thick film techniques were used to prepare aviscous paste composed of 3 parts polymer solution (10% polymer solidsby weight in beta terpiniol) to 1 part TiO₂. The paste was applied toFTO substrates by conventional screen printing techniques. The printedparts were dried at 125° C. and sintered at 400° C. for 1 hr.

A photovoltaic cell was constructed from these sintered samples usingprocedures described previously. The efficiency was 3.5% at a lightintensity of 25 mW/cm² from a xenon.

EXAMPLE 5

This sample was prepared using the techniques described in Example 4.However the sintering temperature for the screen printed substrates wasincreased to 550° C. for 1 hr.

A photovoltaic cell was constructed from these sintered samples usingprocedures described previously. The efficiency was 3.5% at a lightintensity of 25 mW/cm² from a xenon lamp.

EXAMPLE 6

The TiO₂ used in this cell was made as per Example 3b and milledaccording to 3d. The dried and milled TiO₂ was first dispersed inisopropyl alcohol and allowed to settle for 1 hr. The large particlesand agglomerates (>1 micron) were removed by classifying the dispersion.The particle size was measured using a Horiba LA-500 particle sizeanalyzer and found to have an average size of .5 microns.

Samples were next prepared using the procedure described in Example4.These were screen printed two times (drying in between prints) toincrease thickness of the sintered film. The parts were sintered at 450°C. for 1 hr and had an average fired thickness of 8.4 microns.

A photovoltaic cell was constructed from these sintered samples usingprocedures described previously. The efficiency was 3.3% at a lightintensity of 25 mW/cm² from a xenon lamp.

EXAMPLE 7

This sample was made from the classified TiO₂ used in Example 6. Theorganic/inorganic ratio was reduced from 3 parts to 2 parts polymersolution to 1 part TiO₂. A single screen print was applied. The partswere sintered at 450° C. for 1 hr and had an average fired thickness of8.9 microns.

A photovoltaic cell was constructed from these sintered samples usingprocedures described previously. The efficiency was 3.5% at a lightintensity of 25 mW/cm² from a xenon lamp.

What is claimed is:
 1. A process for preparing titanium oxide particles, said process comprising: (a) preparing an aqueous acidic titanium salt solution of from about 10% to about 50% total titanium salt, (b) precipitating the titanium oxide particles by adding the acidic titanium salt solution to an aqueous basic solution containing at least 20% base while maintaining the temperature at from about 20° C. to about 95° C., after initial heating of reaction and dilution, until a pH of from about 2 to about 4 is reached.
 2. A process for preparing doped titanium oxide particles with at least one other metal, said process comprising: (a) preparing an aqueous salt solution of a titanium salt and an aqueous salt solution of a dopant metal salt, (b) adding a major portion of the aqueous titanium salt solution to an aqueous basic solution containing at least 20% base while maintaining the temperature at from about 50° C. to about 95° C. after initial heating of reaction and dilution, to yield a first mixture, (c) combining the remaining titanium salt solution with the dopant metal salt solution to yield a second mixture, and (d) completing the precipitation of the doped titanium oxide particles by adding the second mixture to the first mixture until a pH of from about 2 to about 4 is reached.
 3. The process of claim 1 or claim 2 wherein step (b) comprises maintaining the temperature from about 50° C. to about 85° C.
 4. The process of claim 1 or claim 2 wherein said titanium salt is titanium tetrachloride.
 5. The process of claim 1 wherein said base is ammonium hydroxide.
 6. The process of claim 2 wherein said base is ammonium hydroxide.
 7. The process of claim 1 further comprising a step (c) of recovering said titanium oxide particles by filtration.
 8. The process of claim 1 further comprising a step (c) of filtering the precipitate; washing the precipitate with water until the ionic conductance of water used for washing said precipitate is about 500 micromhos or less; and drying the precipitate.
 9. The process of claim 8 wherein the process further comprises the additional steps of calcining followed by milling until said particles range in size from of 0.1 to 10 microns. 